Row-Overflow Data Exceeding 8 KB
A table can contain a maximum of 8,060 bytes per row. In SQL Server 2005, this restriction is relaxed for tables that contain varchar, nvarchar, varbinary, sql_variant, or CLR user-defined type columns. The length of each one of these columns must still fall within the limit of 8,000 bytes; however, their combined widths can exceed the 8,060-byte limit. This applies to varchar, nvarchar, varbinary, sql_variant, or CLR user-defined type columns when they are created and modified, and also to when data is updated or inserted.
This restriction does not apply to varchar(max), nvarchar(max), varbinary(max), text, image, or xml columns. For more information about the storage of these columns, see Using Large-Value Data Types, Using text and image Data, and Using XML Data.
When you combine varchar, nvarchar, varbinary, sql_variant, or CLR user-defined type columns that exceed 8,060 bytes per row, consider the following:
- Surpassing the 8,060-byte row-size limit might affect performance because SQL Server 2005 Database Engine still maintains a limit of 8 KB per page. When a combination of varchar, nvarchar, varbinary, sql_variant, or CLR user-defined type columns exceeds this limit, the Database Engine moves the record column with the largest width to another page in the ROW_OVERFLOW_DATA allocation unit, while maintaining a 24-byte pointer on the original page. Moving large records to another page occurs dynamically as records are lengthened based on update operations. Update operations that shorten records may cause records to be moved back to the original page in the IN_ROW_DATA allocation unit. Also, querying and performing other select operations, such as sorts or joins on large records that contain row-overflow data slows processing time, because these records are processed synchronously instead of asynchronously.
Therefore, when you design a table with multiple varchar, nvarchar, varbinary, sql_variant, or CLR user-defined type columns, consider the percentage of rows that are likely to flow over and the frequency with which this overflow data is likely to be queried. If there are likely to be frequent queries on many rows of row-overflow data, consider normalizing the table so that some columns are moved to another table. This can then be queried in an asynchronous JOIN operation.
- The length of individual columns must still fall within the limit of 8,000 bytes for varchar, nvarchar, varbinary, sql_variant, and CLR user-defined type columns. Only their combined lengths can exceed the 8,060-byte row limit of a table.
- The sum of other data type columns, including char and nchar data, must fall within the 8,060-byte row limit. Large object data is also exempt from the 8,060-byte row limit.
- The index key of a clustered index cannot contain varchar columns that have existing data in the ROW_OVERFLOW_DATA allocation unit. If a clustered index is created on a varchar column and the existing data is in the IN_ROW_DATA allocation unit, subsequent insert or update actions on the column that would push the data off-row will fail. For more information about allocation units, see Table and Index Organization.
- You can include columns that contain row-overflow data as key or nonkey columns of a nonclustered index.
To obtain information about tables or indexes that might contain row-overflow data, use the sys.dm_db_index_physical_stats dynamic management object.